This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
When hydrocarbons, such as oil and/or natural gas, are produced from underground reservoirs, they must first flow through pores in rock before reaching a wellbore. The ease with which hydrocarbons flow through the rock is known as the permeability of the rock, and is a key factor in the economic success of petroleum production.
It is common in the oil and gas industry to measure permeability on rock samples, such as core samples taken from oil and gas reservoirs. This is one of several laboratory tests providing data for use by engineers in understanding flow in a petroleum reservoir and determining the economics of a petroleum production operation. Other tests include porosity, oil saturation, water saturation, mineral content, and physical properties of oil.
The fundamental unit of permeability is the Darcy (in SI units, 1 Darcy=9.869233×10−13 m2). However, rock in most oil and gas reservoirs has lower permeability than one Darcy, often far lower. Therefore, it has become commonplace to use smaller units such as the following:                milliDarcy (10−3 Darcy) typical of conventional reservoirs;        microDarcy (10−6 Darcy) typical of tight gas reservoirs; and        nanoDarcy (10−9 Darcy) typical of shale formations.        
As mentioned above, shale has exceptionally low permeability, to the extent that it was often considered impermeable by engineers and geologists studying conventional oil and gas reservoirs. When cores are taken for analysis from wells in shale formations, conventional analysis methods may not provide reliable data because the instruments and laboratory techniques were intended for use on rock with much higher permeability values. In addition, calibration standards for laboratory equipment that measures permeability, such as check plugs (permeability calibration standards that are similar in shape to core plugs), are only available for the much higher permeabilities found in conventional oil and gas reservoirs.
Laboratory instruments used to measure permeability are known as permeameters. Many types of permeameters are used in the industry. Most are designed to make measurements on core plugs, which are circular cylinders of rock, typically 1 to 1½ inches in diameter and 1 to 2 inches in length.
Permeameters may be calibrated or checked for accuracy using calibration standards, such as check plugs of known permeability values. In commercial laboratories, check plugs are commonly tested along with actual core samples from a reservoir. For example, one sample out of ten samples tested may be a check plug.
Check plugs may be sensitive to environmental factors such as humidity and mechanical stress. Furthermore, check plugs suffer from the need to empirically measure the permeability of check plugs.
The need still exists for new approaches to calibration of permeameters using check plugs. In particular, there is a need for new approaches due to a lack of available check plugs in the 10-10,000 nanoDarcy range typical of shales or mudstones.